1. Field of the Invention
The present invention relates to a method of and an apparatus for two-way drawing a tubular raw material of thermoplastic resin by a tubular method, and more particularly to improvements in a method of and an apparatus for injecting gas into the tubular raw material.
2. Description of the Related Art
Heretofore, there has been known a method of treating a tubular raw material of thermoplastic resin by a tubular method wherein the tubular raw material is threaded through spaces formed between respective pairs of first pinch rolls and second pinch rolls, and gas is sealed in the tubular raw material which is then heatingly two-way drawn, to thereby produce a film.
As a conventional technique wherein gas is injected into the tubular raw material, there has been known a technique wherein the tubular raw material is cut into linear shapes by a fixed cutter immovably provided at the downstream side of the second pinch rolls in such a manner as to be retractable, and this portion thus cut is spread and inserted thereinto with a nozzle to thereby inject air into the raw material (Japanese Patent Examined Publication No. 59-8341: a conventional example (1). As another conventional technique, there has been known a technique wherein a tubular raw material is threaded through spaces formed between first pinch rolls, second pinch rolls and third pinch rolls having a gap of 1-10 mm therebetween or third pinch rolls each having a multiplicity of grooves in the circumferential direction thereof, and gas is injected into the tubular raw material through an end portion thereof (Japanese Patent Examined Publication No. 57-21450: a conventional example (2).
However, out of the above-described conventional techniques, in the conventional example 1, since the tubular raw material is cut into linear shapes with a single blade, a thick raw material cannot necessarily be cut sufficiently, so that the gas cannot necessarily be injected into the raw material reliably. In addition, the portion thus cut should be spread and the nozzle should be inserted thereinto, whereby there have been presented such disadvantages that insertion of the nozzle into the raw material is not easily effected, workability is low and the size of the nozzle to be inserted is restricted, so that this technique is unsuitable for use in a tubular drawing apparatus which is large-sized, has a large diameter and is operated at a high speed. Further, since the cut portion of the raw material is spread, cracks and wrinkles occur in the raw material in the direction of the longitudinal drawing thereof, thus presenting the disadvantage of that smooth cutting of the raw material with the fixed cutter cannot be effected. In the conventional example 2, since the gas is injected into the raw material being transferred through one end portion thereof, although the tubular raw material can be spread in a small-sized apparatus, the workability is low, a sufficient amount of the gas required for expanding the raw material cannot be injected sufficiently in a large-sized apparatus, thereby presenting the disadvantages of that the workability becomes low and the technique cannot be practicized.